1. Field of the Invention
This invention relates to structures and fabrication methods for organic electronic devices, in particular organic light emitting diodes (OLEDs).
2. Related Technology
To assist in understanding the invention it is helpful to first describe some features of OLED displays and some problems with their fabrication. However it will be appreciated that although embodiments of the invention will be described with specific reference to OLED displays the techniques are more generally applicable to the fabrication of organic electronic devices.
Organic light emitting diodes (OLEDs) are a particularly advantageous form of electro-optic display. They are bright, colorful, fast-switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates. Organic (which here includes organometallic) LEDs may be fabricated using either polymers or small molecules in a range of colors (or in multi-colored displays), depending upon the materials used. A typical OLED device comprises two layers of organic material, one of which is a layer of light emitting material such as a light emitting polymer (LEP), oligomer or a light emitting low molecular weight material, and the other of which is a layer of a hole transporting material such as a polythiophene derivative or a polyaniline derivative.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixellated display. A multicolored display may be constructed using groups of red, green, and blue emitting pixels. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel whilst passive matrix displays have no such memory element and instead are repetitively scanned to give the impression of a steady image.
FIG. 1 shows a vertical cross section through an example of an OLED device 100. In an active matrix display part of the area of a pixel is occupied by associated drive circuitry (not shown in FIG. 1). The structure of the device is somewhat simplified for the purposes of illustration.
The OLED 100 comprises a substrate 102, typically 0.7 mm or 1.1 mm glass but optionally clear plastic, on which an anode layer 106 has been deposited. The anode layer typically comprises around 150 nm thickness of ITO (indium tin oxide), over which is provided a metal contact layer, typically around 500 nm of aluminum, sometimes referred to as anode metal. Glass substrates coated with ITO and contact metal may be purchased from Corning, USA. The contact metal (and optionally the ITO) is patterned as desired, and so that it does not obscure the display, by a conventional process of photolithography followed by etching.
A substantially transparent hole transport layer 108a is provided over the anode metal, followed by an electroluminescent layer 108b. Banks 112 may be formed on the substrate, for example from positive or negative photoresist material, to define wells 114 into which these active organic layers may be selectively deposited, for example by a droplet deposition or inkjet printing technique. The wells thus define light emitting areas or pixels of the display.
A cathode layer 110 is then applied by, say, physical vapour deposition. A cathode layer typically comprises a low work function metal such as calcium or barium covered with a thicker, capping layer of aluminum and optionally including an additional layer immediately adjacent the electroluminescent layer, such as a layer of lithium fluoride, for improved electron energy level matching. Mutual electrical isolation of cathode lines may achieved through the use of cathode separators (element 302 of FIG. 3b). Typically a number of displays are fabricated on a single substrate and at the end of the fabrication process the substrate is scribed, and the displays separated before an encapsulating can is attached to each to inhibit oxidation and moisture ingress.
Organic LEDs of this general type may be fabricated using a range of materials including polymers, dendrimers, and so-called small molecules, to emit over a range of wavelengths at varying drive voltages and efficiencies. Examples of polymer-based OLED materials are described in WO90/13148, WO95/06400 and WO99/48160; examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343; and examples of small molecule OLED materials are described in U.S. Pat. No. 4,539,507. The aforementioned polymers, dendrimers and small molecules emit light by radiative decay of singlet excitons (fluorescence). However, up to 75% of excitons are triplet excitons which normally undergo non-radiative decay. Electroluminescence by radiative decay of triplet excitons (phosphorescence) is disclosed in, for example, “Very high-efficiency green organic light-emitting devices based on electrophosphorescence” M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, and S. R. Forrest Applied Physics Letters, Vol. 75 (1) pp. 4-6, Jul. 5, 1999. In the case of a polymer-based OLED layers 108 comprise a hole transport layer 108a and a light emitting polymer (LEP) electroluminescent layer 108b. The electroluminescent layer may comprise, for example, around 70 nm (dry) thickness of PPV (poly(p-phenylenevinylene)) and the hole transport layer, which helps match the hole energy levels of the anode layer and of the electroluminescent layer, may comprise, for example, around 50-200 nm, preferably around 150 nm (dry) thickness of PEDOT:PSS (polystyrene-sulphonate-doped polyethylene-dioxythiophene).
FIG. 2 shows a view from above (that is, not through the substrate) of a portion of a three-color active matrix pixellated OLED display 200 after deposition of one of the active color layers. The figure shows an array of banks 112 and wells 114 defining pixels of the display.
FIG. 3a shows a view from above of a substrate 300 for inkjet printing a passive matrix OLED display. FIG. 3b shows a cross-section through the substrate of FIG. 3a along line Y-Y′.
Referring to FIGS. 3a and 3b, the substrate is provided with a plurality of cathode undercut separators 302 to separate adjacent cathode lines (which will be deposited in regions 304). A plurality of wells 308 is defined by banks 310, constructed around the perimeter of each well 308 and leaving an anode layer 306 exposed at the base of the well. The edges or faces of the banks are tapered onto the surface of the substrate as shown, typically at an angle of between 10 and 40 degrees. The banks present a hydrophobic surface in order that they are not wetted by the solution of deposited organic material and thus assist in containing the deposited material within a well (although polar or non-polar solvents may be employed generally the solvents used have some polarity). This is achieved by treatment of a bank material such as polyimide with an O2/CF4 plasma as disclosed in EP 0989778. Alternatively, the plasma treatment step may be avoided by use of a fluorinated material such as a fluorinated polyimide as disclosed in WO 03/083960.
As previously mentioned, the bank and separator structures may be formed from resist material, for example using a positive (or negative) resist for the banks and a negative (or positive) resist for the separators; both these resists may be based upon polyimide and spin coated onto the substrate, or a fluorinated or fluorinated-like photoresist may be employed. In the example shown the cathode separators are around 5 μm in height and approximately 20 μm wide. Banks are generally between 20 μm and 100 μm in width and in the example shown have a 4 μm taper at each edge (so that the banks are around 1 μm in height). The pixels of FIG. 3a are approximately 300 μm square but, as described later, the size of a pixel can vary considerably, depending upon the intended application.
Techniques for the deposition of material for organic light emitting diodes (OLEDs) using ink jet printing techniques are described in a number of documents including, for example, T. R. Hebner, C. C. Wu, D. Marcy, M. H. Lu and J. C. Sturm, “Ink-jet Printing of doped Polymers for Organic Light Emitting Devices”, Applied Physics Letters, Vol. 72, No. 5, pp. 519-521, 1998; Y. Yang, “Review of Recent Progress on Polymer Electroluminescent Devices,” SPIE Photonics West: Optoelectronics '98, Conf. 3279, San Jose, January, 1998; EP O 880 303; and “Ink-Jet Printing of Polymer Light-Emitting Devices”, Paul C. Duineveld, Margreet M. de Kok, Michael Buechel, Aad H. Sempel, Kees A. H. Mutsaers, Peter van de Weijer, Ivo G. J. Camps, Ton J. M. van den Biggelaar, Jan-Eric J. M. Rubingh and Eliav I. Haskal, Organic Light-Emitting Materials and Devices V, Zakya H. Kafafi, Editor, Proceedings of SPIE Vol. 4464 (2002). Ink jet techniques can be used to deposit materials for both small molecule and polymer LEDs.
A volatile solvent is generally employed to deposit a organic electronic material, with 0.5% to 4% dissolved solvent material. This can take anything between a few seconds and a few minutes to dry and results in a relatively thin film in comparison with the initial “ink” volume. Often multiple drops are deposited, preferably before drying begins, to provide sufficient thickness of dry material. Solvents which may be used include cyclohexylbenzene and alkylated benzenes, in particular toluene or xylene; others are described in WO 00/59267, WO 01/16251 and WO 02/18513; a solvent comprising a blend of these may also be employed. Precision ink jet printers such as machines from Litrex Corporation of California, USA are used; suitable print heads are available from Xaar of Cambridge, UK and Spectra, Inc. of NH, USA. Some particularly advantageous print strategies are described in the applicant's UK patent application number 0227778.8 filed on 28 Nov. 2002 (and the corresponding PCT publication WO 2004/049466).
Inkjet printing has many advantages for the deposition of materials for organic electronic devices but there are also some drawbacks associated with the technique. However it has been found that dissolved organic electronic material deposited into a well with shallow edges dries to form a film with a relatively thin edge. FIGS. 4a and 4b illustrate this process.
FIG. 4a shows a simplified cross section 400 through a well 308 filled with dissolved material 402, and FIG. 4b shows the same well after the material has dried to form a solid film 404. In this example the bank angle is approximately 15° and the bank height is approximately 1.5 μm. As can be seen a well is generally filled until it is brimming over. The solution 402 has a contact angle θc with the plasma treated bank material of typically between 30° and 40° for example around 35°; this is the angle the surface of the dissolved material 402 makes with the (bank) material it contacts, for example angle 402a in FIG. 4a. As the solvent evaporates the solution becomes more concentrated and the surface of the solution moves down the tapering face of a bank towards the substrate; pinning of the drying edge can occur at a point between the initially landed wet edge and the foot of the bank (base of the well) on the substrate. The result, shown in FIG. 4b, is that the film of dry material 404 can be very thin, for example of the order of 10 nm or less, in a region 404a where it meets the face of a bank.
We have previously described, in UK Patent Application No. 0402559.9, filed 5 Feb. 2004, published as WO 2005/076386, the use of undercut banks which the have the effect of pulling the solution towards the edge of a well, thus helping to obtain a more uniform fill. However such banks can be tricky to fabricate and generally use negative photoresist which is expensive and sensitive to process conditions. There is therefore a need for further improved techniques which are more suitable for use with positive photoresists.
Another difficulty arises with larger pixels (wells), for example wells with an opening of 240 μm to 260 μm used to provide a 300 μm pixel pitch. The volume of an ink drop is proportional to the cube of a characteristic length for the drop whereas surface coverage is proportional to the square of a pixel dimension and because of this for a given ink dilution too much material is deposited into a large pixel so that more dilute ink is needed. For example for large pixels and a desired PEDOT film thickness of 80 nm an ink concentration of approximately one percent may be employed, but it is difficult to get one percent ink to spread out, wet and fill a large pixel. This makes the fabrication of pixels more than 500 μm square difficult because, say, a full-to-brimming pixel results in a 120 nm thickness film. Moreover it is expensive to alter the ink dilution in a production process.
Generally the base of a pixel well comprises ITO, which has a low contact angle, typically less than 10 degrees (for example five to seven degrees) and thus provides relatively good (hydrophilic) wetting. However, particularly with larger pixels, wetting is never perfect and rather than a deposited droplet having a circular shape, a deposited droplet generally has a more ragged edge because the solvent tends to be pinned at points within the well base. As previously mentioned, because the contact angle of the solvent on the bank is relatively high as more solvent is added to a well the droplet height tends to increase rather than the solvent moving up the bank and, on drying, surface energy tends to pull the solution away from the edge of the well. This is particularly a problem with PEDOT deposition where the thin edge can result in direct contact between the cathode (ITO) and overlying light emitting polymer (LEP) resulting in a defective or reduced efficiency pixel. In EP 0993235 Seiko Epson have aimed to address this problem by depositing a dielectric layer over the anode at the inside edge of the base of a pixel well, but this has the drawback of reducing the effective pixel area by up to 20 percent when the need for alignment tolerance is taken into account.
There is therefore a need for improved organic electronic device structures and fabrication techniques which address these problems and, in particular, help to spread out organic electronic material in a solvent-based deposition process.
According to a first aspect of the present invention there is therefore provided a organic electronic device structure, the structure comprising: a substrate; a base layer supported by said substrate and defining the base of a well for solvent-based deposition of organic electronic material; one or more spacer layers formed over said substrate; a bank layer formed over said spacer layer to define a side of said well; and wherein an edge of said well adjacent said base layer is undercut to define a shelf over said substrate, said shelf defining a recess to receive said organic electronic material.
Preferably the underside of the shelf is substantially horizontal and spaced away from the substrate by a distance defined by the one or more spacer layers. The shelf may be defined by the bank or one or more shelf layers may be included in the structure between the spacer layer and the bank layer, to define the shelf. Preferably the shelf layer comprises the dielectric layer but, in embodiments, the shelf layer may include a metal layer. It will be appreciated from the embodiments described later that the shelf and/or spacer layers are generally provided by layers which are already present for fabrication of the device such as metal, oxide and/or doped or undoped silicon layers. For an active matrix display device a thin film transistor (TFT) is associated with each pixel and then the spacer layer may be formed by part of a doped and/or undoped amorphous silicon layer used for fabrication of the TFT, or an oxide layer. Likewise the shelf layer may also conveniently be formed by one of the layers which are in any case deposited during fabrication of the TFT such as a silicon nitride dielectric and/or passivation layer.
Preferably the recess under the shelf is configured to permit contact between the spacer layer and the organic electronic material—that is the recess exposes an edge of the spacer layer. This assists in pinning the solvent to the edge of the well. Thus in preferred embodiments (where the solvent is at least partially polar) the spacer layer comprises a hydrophilic material such as silicon, silicon monoxide, silicon dioxide, silicon oxynitride or the like. Optionally the spacer layer may be treated to make it (more) hydrophilic.
In embodiments using a hydrophilic material for the spacer layer helps to de-couple desired attributes for PEDOT and LEP wetting since the PEDOT wets the hydrophilic spacer layer thus allowing the wetting angle of the LEP on the bank to be adjusted substantially independently (since PEDOT wetting is dominated by the hydrophilic material). For example generally the bank resist is hydrophobic with a wetting angle of greater then 90 degrees for the LEP solvent, but this may be reduced to less than 90 degrees, 60 degrees or even 30 degrees, for example to provide improved wetting of the LEP to the bank material. Because the PEDOT solvent runs under the shelf and is pinned by the hydrophilic layer the risk of shorting across the PEDOT layer is drastically reduced. It will be appreciated that although reference has generally been made to the spacer layer providing a hydrophilic edge for the PEDOT solvent pinning (because solvents with some polarity tend to be employed), more generally good wetting of the solvent to the exposed spacer layer edge is desirable as defined, for example, by a low contact angle such as 15 degrees, 10 degrees or less.
It will be appreciated that with embodiments of the above structure the bank layer can taper towards the substrate in the usual direction (getting thinner towards the substrate as the well side is approached), and therefore a bank may be defined using positive photoresist.
In some preferred embodiments the structure forms part of an OLED display device such as an active matrix display pixel. In this case the base layer generally comprises a transparent anode layer such as ITO and the organic electronic material which is deposited into the well comprises a first layer of conducting (whole) transport material such as PEDOT overlying which is a second layer of light emitting material, for example light emitting polymer, small molecule material, dendrimer-based material or the like. The recess under the shelf is then occupied by the first layer of organic electronic material (for example the PEDOT), preferably substantially fully occupied by this material, and the second light-emitting layer overlies the first layer and may also ride partially up the bank. In other embodiments the light-emitting layer may also lie under the shelf and be pinned at the well edge by a second spacer layer which may, for example, be tuned according to the solvent used to deposit the light-emitting layer to provide good wetting. In one such embodiment the first spacer layer comprises undoped (amorphous) silicon and the second spacer layer doped (amorphous) silicon, both used in the fabrication of an active matrix TFT transistor and hence convenient to deposit for pinning at the well edge.
In a related aspect the invention provides a method of fabricating a organic electronic device on a substrate, the method comprising fabricating one or more base layers on said substrate; fabricating one or more spacer layers on said one or more base layers; depositing bank material over said one or more spacer layers; etching said substrate to define a well with an undercut shelf defining a recess at its base; and depositing organic electronic material into said well.
Preferably the etching comprises at least partially self-aligned etching. In this way a mask used to define the bank may also be used for etching the undercut shelf to expose the spacer layer for, in a partially self-aligned device, for etching the shelf layer.
In a further aspect the invention provides a method of forming a droplet deposition well in a structure for droplet-deposition-based fabrication of a organic electronic device, the method comprising: depositing onto a substrate, a layer of hydrophilic material; depositing a layer of bank material over said layer of hydrophilic material; patterning said layer of bank material to define banks forming one or more of said droplet position wells; and etching said layer of hydrophilic material in a self-aligned process using said patterned layer of bank material as a resist.
In embodiments this method avoids the need for two separate mask steps, one for the bank material and a second for the hydrophilic (or spacer) layer. The skilled person will understand that in general the substrate to which the method is applied will have been purchased or prepared with an initial, underlying layer of transparent conductor, such as ITO. In some preferred embodiments of the method the bank material comprises resist, preferably positive resist. Preferably a bank has only one layer of bank material (which is preferably hydrophobic), and only one layer of hydrophilic material (such as oxide).
In some preferred embodiments of the method the hydrophilic material comprises dielectric material, in particular SiO2, although other dielectric materials such as silicon nitride and silicon oxynitride or even resist may also be employed. In other embodiments the hydrophilic material comprises a hydrophilic metal such as aluminum, chrome or molychrome. In such embodiments the metal may be, for example, anode metal formed over the ITO to reduce the anode track resistance. In embodiments of an OLED device fabricated using this method the metal can be exposed to organic electronic material, in particular PEDOT, which is afterwards deposited in the well. However where the metal is a poor electron injector (has a high work function) for the relevant material (PEDOT), this contact does not significantly the operation of the device because, in effect, it behaves substantially as an insulator.
The self-aligned etch stage, in which the bank resist acts as a mask, may be either isotropic or anisotropic. In preferred embodiments the etching comprises plasma etching. An isotropic etch undercuts the hydrophilic layer (which thus acts as a spacer between the substrate and the overlying bank layer); an anisotropic etch cuts substantially vertically down through the edge of the hydrophilic later where the bank edge (which is generally tapered) terminates. In the case of an isotropic etch a dry etch, in particular a plasma gas etch may be employed, which is self-limiting within the undercut, allowing the depth of the undercut to be controlled. Alternatively a wet etch may be employed, which continues to etch so long as the etchant is present. For an anisotropic etch a dry plasma etch is preferred.
For an undercut embodiment of the device structure the hydrophilic (spacer) layer may have a thickness of less than 500 nm, for example between 50 nm and 200 nm, and some in embodiments around 100 nm. In other embodiments, where the hydrophilic layer provides (effective) insulation to help reduce shorting at the base of the bank edge (where the deposited organic electronic material tend to be thinner because of solvent drying effects), the hydrophilic layer may be thinner, for example less than 100 nm, 50 nm, 10 nm or 5 nm. The limiting thickness is determined by the desire to form a continuous insulating film, and may be around 2 nm for SiO2. An anisotropic etch is preferred for embodiments of the method which create an insulating shelf at the base of a well, as this substantially prevents undercutting, thus reducing the quantity of bank material to be removed to leave such a shelf.
In some particularly preferred embodiments a resist stripping procedure, preferably a plasma ashing procedure such as an O2 plasma ash is performed after the etching. This removes part of the base of the (tapered) bank where it is thinnest (and also reduces the overall thickness of the bank), that is adjacent the base of the well, to expose the hydrophilic material adjacent the base of the well. This enlarges the aperture of the bank material so that it is greater than that in the hydrophilic layer. The exposed part of the hydrophilic layer acts as an insulating spacer, as previously mentioned, to help to prevent shorts at the edge of a well base. In particular it attracts the hydrophilic PEDOT solution, which effectively attaches to the exposed part of this material, pinning the edge of a droplet of such deposited material. Moreover, because the PEDOT solution is confined in this way, the surface energy properties of the layer of bank material can be separately tuned towards a desired character for a subsequently deposited layer of material, such as a layer of light emitting polymer (LEP). In the case of bank resist, for example, this may be done by treating the bank material with a CF4 plasma to make it hydrophobic, for better LEP confinement. (This “tuning” has little effect on the hydrophilic property of the underlying oxide, although there is a small “contamination effect”). Alternatively a “teflonised” or fluorinated resist may be employed to achieve a hydrophobic bank character. Thus these embodiments of the method broadly speaking allow the different desired surface energy treatments (hydrophilic and hydrophobic) for solution deposition of, say PEDOT and LEP, to be decoupled. Moreover, the high bank contact angle of, say a solution of PEDOT in water, which may be 90°-110°, helps keep the PEDOT off the bank and therefore contain this material.
Thus in a further aspect the invention provides a method of forming a droplet deposition well in a structure for droplet-deposition-based fabrication of a organic electronic device, the method comprising: depositing onto a substrate, a layer of hydrophilic material; depositing a layer of resist material over said layer of hydrophilic material; patterning said layer of resist material to define banks forming one or more of said droplet deposition wells; patterning said layer of hydrophilic material to remove said hydrophilic material from at least part of a base area of said one or more droplet deposition wells; and using a resist stripping procedure to expose part of an upper surface of said patterned layer of hydrophilic material adjacent a base of a said bank forming one or more of said droplet deposition wells.
In embodiments providing a lower layer which juts out beyond the bank, and which has a similar surface energy to underlying ITO, can help yield and uniformity without a significant effect on cost and aperture ratio. The structure which is subjected to the resist stripping (ashing) need not have been formed by a self-aligned process but could, for example, have been formed using a two-mask process.
In a related aspect the invention provides a organic electronic device fabricated using a method as described above. In particular such a device comprises a substrate bearing a patterned layer of hydrophilic material beneath a plurality of droplet deposition wells filled with organic electronic material, wherein part of an upper surface of said patterned layer of hydrophilic material adjacent a base of a said bank forming one or more of said droplet deposition wells is exposed to said organic electronic material.
In a still further aspect the invention provides a organic electronic device structure, the structure comprising: a substrate; and a bank layer supported by said substrate and defining a well for solvent-based deposition of organic electronic material; and wherein said structure further comprises a fillet layer patterned to define a fillet at an inside edge of said well and at the base of said well.
Preferably the fillet comprises hydrophilic material such as an oxide and/or nitride of silicon. The fillet layer may conveniently comprise a layer which also forms some other part of the organic electronic device such as an oxide layer of a thin film transistor forming part of or associated with the device.
In one embodiment the fillet overlies a part of the bank which tapers down towards the substrate; in another set of embodiments one or more layers are patterned to define a step at the inside edge of the well and the fillet abuts the step. For example in preferred embodiments the well has a base layer, such as an anode or ITO layer and a step layer (which again may be provided by “re-using” an existing layer of the device) is provided between the base layer and the substrate to provide a step change in height in the base layer adjacent the inside edge of the well. In this case the fillet abuts this step in the base layer. In some embodiments a double step is defined in the base layer using two (or more) underlying “step” layers to provide a taller spacer stack under the ITO and thus a larger fillet area for improved solvent pinning at the edge of the well. In embodiments the one or more step layers may comprise one or more of a metal layer, an undoped silicon layer, a doped silicon layer, and a second metal layer. These layers may, for example, already be present as part of an existing fabrication process for a thin film transistor associated with a pixel of an OLED display device. Preferably in such a device the bank layer comprises positive photoresist and is conventionally tapered towards the substrate. The deposited layers of organic electronic material in this case may comprise a conductive (whole transport) layer and an overlying light-emitting layer.
In another aspect the invention provides a method of fabricating a organic electronic device on a substrate bearing at least one well for solvent-based deposition of organic electronic material, the method including depositing a fillet layer and anisotropically etching said fillet layer to define a fillet at an inside edge of said well prior to solvent-based deposition of said organic electronic material for fabricating said device.
Preferably the fillet material is selected or treated such that it is wetted by a solvent or solvent mix used to deposit the organic electronic material. Preferably such wetting provides a contact angle between the fillet and the solvent or solvent mix of less than 15 degrees, more preferably less than 10 degrees.